1) Field of the Invention
The field of this invention relates to testing equipment and more particularly to an apparatus which is usable to determine the transfer of a substance through a membrane into a liquid placed in contact with the opposite side of the membrane.
2) Description of the Prior Art
Percutaneous absorption test cells are used in the study of kinetics of the partition coefficients defining the passage and equilibrium states of components into and through a barrier separating two dissimilar substances. A typical test cell is known as the Franz cell. The Franz cell is in the form of a container with an upper half separated from a lower half by a porous membrane comprising a barrier. A clamping arrangement is located between the upper half of the lower half with the barrier in the form of a thin sheet of material to be placed and clamped tightly in a stretched configuration by a clamping arrangement.
The lower compartment of the Franz cell is completely filled with a receptor media in contact with the barrier. In the upper compartment there is placed a donor substance which may comprise a solid, semi-solid, gas or liquid. The receptor media normally comprises water, a buffered solution or a saline solution, but may consist of any polar or non-polar liquid, semi-liquid or gas with or without surfactants added. Connecting the lower compartment is a sample port and it is through this sample port that aliquots are to be removed. An aliquot is defined as an exact subvolume of the overall volume of the receptor media. A prior art type of diffusion cell has been obtained by the present inventor and carries U.S. Pat. No. 5,198,109, issued Mar. 30, 1993.
While generally useful in the whole field of physical chemistry, Franz cells have become particularly useful in the health care field. Levels of epidermal exposure to pesticides, chemicals, ointments and cosmetics are important in the field of environmental science. The rate of release and absorption of ingredients for medicinal skin patches is essential to determine the size, dosage amount and characteristics of the patch.
Within a Franz cell, typically the test is conducted by placing the donor substance against the total area of the barrier exposed to the upper compartment. A suitable receptor media, such as a saline solution, is placed in the lower compartment in contact with the barrier. Within this lower compartment is located a stirring device. The function of the stirring device is to homogeneously intermix the portion of the donor media that permeates the membrane into the receptor chamber.
The kinetics of membrane transfer involves more than the pore size of the membrane. Membranes have varying degrees of hydrophilic and hydrophobic characteristics. This is particularly true of skin. There are three layers of skin with varying characteristics in allowing the passage of fluids. While synthetic membranes are generally chosen for transfer studies with diffusion cells, natural skin, either human or animal, is used extensively within Franz cells.
Animal and human membranes may inadvertently release particulate matter into the receptor compartment. This particulate matter typically consists of organic integumentary particles of highly viscous aggregates ranging from diameters of one hundred microns to string-like particles of one thousand or more microns in length. Such particulates may clog the sample withdrawal tube and prevent consistent withdrawal rates particularly in an automated sampling system.